Fuel cells have been used as a power source in many applications and have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies, which comprise the catalyzed electrodes, are relatively expensive to manufacture and require certain controlled conditions in order to prevent degradation thereof.
Efficient operation of a fuel cell depends on the ability to effectively disperse reactant gases at catalytic sites of the electrode where reaction occurs. In addition, effective removal of reaction products is required so as to not inhibit flow of fresh reactants to the catalytic sites. Therefore, it is desirable to improve the mobility of reactant and product species to and from the MEA where reaction occurs
To improve the mobility of reactant and product species to and from the MEA where reactions occur, a diffusion structure which enhances mass transport to and from an electrode in a MEA of a fuel cell is used. The diffusion structure cooperates and interacts with an electrode at a major surface of the electrode opposite the membrane electrolyte of the cell, therefore, electrical and heat conductivity are required. The diffusion structure is typically a composite diffusion medium which facilitates the supply of reactant gas to the electrode. The diffusion structure also facilitates movement of water and the products of the reactions. The typical diffusion structure includes a characteristic bulk layer having two or more portions, such as a PTFE coating and/or a microporous layer, each with various properties, including hydrophobicity and surface energy. The bulk layer is also usable alone to function as a diffusion structure. However, it is preferably combined within an absorption layer and a desorption layer on respective sides of the bulk layer to form a preferred diffusion structure. The diffusion structure, either the bulk layer alone or combined with other layers, is hereinafter referred to as a diffusion media. See for example U.S. Pat. No. 6,350,539 issued to the assignee of the present application. The diffusion media is positioned between the MEA and the cathode or anode flow channels of an individual fuel cell.
The quality of a diffusion media is hard to control due to there only being a few tests indicating the performance of a diffusion media. During the manufacturing of a diffusion media, there can be several steps. A first step can include a hydrophobization step, such as teflonization of the bulk layer (applying PTFE to the bulk layer) or coating the bulk layer with other low surface energy substance(s) and a second step can include coating the hydrophobized bulk layer with a microporous substrate. To date, the PTFE content is routinely checked by weight and/or by fluorine mapping using a scanning electronic microscope (SEM). The coating can also be visually checked. The weight check is not very significant due to averaging the weight gain for the whole sheet (bulk layer). That is, the amount of PTFE at any given location is not known, rather, the total amount of PTFE on the bulk layer is determined and used to calculate an average PTFE content on a per unit basis. Thus, the weight check can not identify specific areas of the diffusion media that have an undesirable PTFE content nor provide a quantitative measure indicative of performance and surface energy. Fluorine mapping is not always desirable because it is expensive and time consuming. The visual test can identify contrasting dark and light spots in the diffusion media coating which are indicative of problems during fabrication. The visual test, however, is a qualitative check that is only useful in spotting large area defects. For controlling the microporous layer, the weight and the gas flow through the diffusion media can be checked for quality control. Additionally, the thickness of the microporous layer can be used for quality assurance purposes. These methods, however, do not appear to adequately relate to the properties effecting the performance of the diffusion media in a fuel cell. Thus, an improved method for quality control is needed.